As is the case with most problems that seem complicated I appear to be chasing multiple issues at once. First, thank you Mike for the picture and that is the approach I am taking - except right now I have an air compressor and regulator providing my test pressures.

I have found that the dedicated wire for the gauge just stopped inside the factory harness! I pulled back the original electrical tape - and I say that with confidence because over the top of the tape is the tag C5TB 10A665E WCC - and the wire was cut right in the middle of the harness. And the electrical tape was one contiguous piece - all the way to the end at the gauge and the terminal lugs.

I'll keep you updated as I discover more.